was/Output: do not cancel PipeEvent in WriteEventCallback()
was/Output: do not cancel PipeEvent in WriteEventCallback()

Cancel it only if Istream::Read() did not deliver any data.  This
eliminates yet more unnecessary epoll_ctl() calls.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy